To provide a transaction support method, device and system that support a transaction between a purchaser and a seller.

The transaction support method, which supports a transaction of merchandise of a transaction amount fixed in the future between a purchaser and a seller, comprises a step of accepting a provisional transaction amount of the transacted merchandise from the purchaser or the seller, a step of deciding an appraisal amount of the sales credit of the merchandise according to the provisional transaction amount, a step of notifying the seller of the appraisal amount, a step of accepting approval when the seller approves the appraisal amount, a step of comparing an actual transaction amount and an amount corresponding to the appraisal amount when the actual transaction amount of the merchandise is fixed to compute an amount corresponding to their difference, and a step of storing the amount corresponding to the difference as information for future buying of sales credit from or financing to the seller.